Ophthalmic lens



E. D. TILLYER OPHTHALMIC LENS July 26, 1938.

A ,ED

Filed Dec. '7, 1935 INVENTOR BY 619/? 12 T/u. YE}? 93 f/ ZL MAX ATTORNPatented July 26, 1938 UNITED STATES I PATENT OFFICE,

American Optical Company,

Southbrldge.

Masa, a voluntary association of Massachusetts Application December 7,1935, Serial No. 53,351

3 Claims.

This invention relates to improvements in bifocal or multifocal lensesand to an improved process of making the same and relates moreparticularly to bifocal or multifocal lenses used for the equalizationof the mental impressions of size in the two eyes throughout thecorresponding focal fields of the lenses without materially changing therequired focal powers of said fields,

said impressions being also referred to in the art as ocular images.

This application is a continuation in part of my copending application,Serial No. 653,058, filed January 23, 1933, which has become Patent No.2,024,552, dated December 17, 1935.

One of the principal objects of the invention is to provide improvedmeans and method for forming lenses of the above characterwith fields ofthe same or different powers and/or magnifications.

Another object of the invention is to provide means of separating in alens or lens systems of the above character. the size and focal powerfactors so that the said factors may be made the same or varied withrespect to each other in the respective focal fields of the lens wherebythe said lens may be made in a blank form, so one surface is left forthe impression of the prescriptive focal power required in eachrespective focal field, the remaining parts giving the true size effectindependently of the respective prescriptive surfaces.

Other objects are to provide lenses of this character having increasedpower for the reading distance and/or having different power and/ormagnification for the reading portion from that of the distance portion,and/or having the reading portion of the same power as the distanceportion but of different magnification, that is to say, lenses having adistance portion including means for change of size of image over theprescription requirements without change of focus thereof and anotherportion different from the first portion, and/or having a readingportion of different power than the distance power but of substantlallythe same magnification.

Another object of the invention is to provide a one piece type bifocalor multifocal lens having the above characteristics.

Another object of the invention is to provide a fused type one elementlens, that is, a lens having its major portion formed from a singlepiece of glass, and having the above characteristics.

Another object of the invention is to provide an ophthalmic correctionfor the two eyes of a patient whereby the focal power factors of therespective lenses in each of the respective focal fields of said lensesare to the prescriptive requirements of the patient and also providemeans for balancing the ocular image or mental impression sizedifferences of the eyes in each of the respective fields of the lenseswith substantially no change in the focal power factors of therespective focal fields of said lenses.

Other objects and advantages of the invention will become apparent fromthe following description taken in connection with the accompanyingdrawing. It will be apparent that many changes in the arrangement ofparts, details of construction and steps of the process may be madewithout departing from the spirit of the invention as expressed in theaccompanying claims. I, therefore, do not wish to be confined to theexact matters shown and described as the preferred forms have been shownby way of illustration only.

Referring to the drawing:

Fig. I is a front view of a lens embodying the invention;

Fig. 11 is a cross sectional view taken on line 1III of Fig. I, showingone form of the invention;

Fig. III is a cross section showing another form of the invention;

Fig. IV is a cross section showing another form of the invention;

Fig. V is a diagrammatic view in cross section, showing the relation ofthe lens to the eye;

Fig. VI is a cross section of another form of the invention;

Fig. VII is a sectional view of a one piece type single element bifocalor multifoeal lens embodying the invention; and

Fig. V111 is'a sectional view similar to Fig. VII of a fused typebifocal or multifocal lens embodying the invention.

In the past, eye corrections embraced the corrections for sphere,cylinder and prism, either separately, in various combinations orembodying all of said corrections. Recently, a fourth correction hasbeen added to these, namely, a correction for the difference in size ofimage of the two eyes or in different meridians of an eye without achange of the focus of the prescription requirements of the eye or eyes.The inclusion of this fourth element has introduced difficultles intothe art of lens making not hitherto encountered; either additional lenssurfaces are required or a modification of the various spherical,cylindrical or toric curves now in use may be required.

The new problem introduces practically a new art in eye examination andin the art of making lenses. These diiilculties are increased where itis attempted to provide the new form of lens with-fields for bothdistance and reading corrections, as it has been found by pastexperience and by actual test that the eyes of a patient may have adifferent ratio of size error when looking at a near object than whenlooking at a distant obiect and that this size difference may beentirely different from that introduced by the change of co power in thenear vision portion of the lens. This, therefore, necessitates theprovision of lenses having two or more diflerent focal fields fordifferent object distances wherein the power factors and themagnification factors of the different respective focal fields may bealtered independently of each other and without the altering of onefactor bringing about a change in the other factor in the finishedlenses. It is, therefore, one of the principal objects of this inventionto provide lenses of the above character with different focal fields andto provide a practicable and economical process for making the samewhereby all of the desired characteristics of the lenses may beincorporated in an ophthalmic correction embodying one or more lenselements in the lens system or systems producing the final prescriptiverequirements.

Referring to the drawing wherein like characters of reference indicatelike parts throughout:

In Fig. V, I have shown an elementary lens of two parts designed tochange the size of image of the focal prescription requirements withoutchange of power thereof. The eye is shown-at 3. The element l is theordinary prescription lens having the surfaces i5 and i6 designed in theusual prior art way for corrections of sphere, cylinder and prism, one,all or any. The element 2 is the element thatprovides the change in sizeof image from the focal prescription requirement with practically nochange of power thereof. It has the surfaces i1 and I8 arranged asfollows: The relationship of the surfaces i1 and I8 is such as toprovide substantially no optical power but a magnification. Thesesurfaces maybe fiat, spherical, toric or cylindrical. The amount ofchange in size depends upon the curvatures of the two surfaces and thethickness of the element. Where change in size of the two majormeridians is desired, the spherical surfaces are used; when change inone meridian is desired only, the cylindrical surfaces are used. In sucha lens element the two surfaces in order to produce no optical power arenearly concentric.

When the concave side of the element is placed nearest the eye, the sizeof the image is increased; when the convex side of the element isnearest the-eye, the size of the image is decreased; the former ispreferred. The desired change in size is obtained by the opticalrelationship or shape of the two surfaces of the element and. thethickness thereof. The manification in a no power optical element is dueto the bending or curving of the element. If a distant object is viewedthrough a plane parallel, the efiect of this plane parallel isnegligible.

If, however, we bend this plane parallelas happens when it is ground ondifi'erent base curves, a magnification will be produced. The element 2is shown curved or bent ,to give the desired amount of magnification.

For description of lenses of this nature, see article entitled Lensesfor changing the size and shape of dioptric images by Ames, Gliddon andOgle of the Department of Research in Physiological Optics, DartmouthMedical School, Hanover, New Hampshire, contained in 9. pamphlet reprintfrom the Annals of the Distinguished Service Foundation of Optometry,Boston, Massachusetts, 1932, page 27.

The method of constructing a "size lens, having focal power and inaddition thereto, a size magnification independent of the magnificationdue to power, is old in the art, being set forth in United StatesLetters Patent No. 1,933,578 Ames, November 7, 1933.

The method of obtaining the thickness, sur-- faces, separations and lenscharacteristics are set forth in this patent with the necessary formulaand examples, etc, The lens of the said Ames patent, is the lens of thepublication referred'to above. The lens produced gives the requiredaxial focal power and the required "size" manification. As explained inthe said patent, the characteristics are, the distance from the eye,thedistance to the object, the thickness and separations and thecurvatures of the surfaces. The "size" element is a function of the formor shape of the lens and the thickness, the Dower element is therelationship of the surfaces, one -to the other, as usual in prior artlenses.

The first step in the invention is to calculate the basic size" andpower lens for a given distance of object as described in saidpublication and patent. This is usually the major field of the lens ofthe invention.

For the purpose of providing an additional field a second focal field 4for the reading distance is placed on the lens, the other portion of thelens being arranged for distance vision.

The arrangement of the two fields will be dependent on the opticalrequirements for each field. In Figs. II to IV, inclusive, I have shownthe surfaces of the distance field elements as fiat. It will beunderstood that these surfaces will be curved in most instances ascalled for by the various prescriptive requirements as well understoodin the art. I have also shown these lens elements as separated. It willbe understood that these elements may be separated or abutted oneagainst the other and secured together by cement, or fusing orotherwise, or if modified surfaces are used they may be made in onepiece. The reading field 4 may be made with a segment cemented or fusedon or may be ground in one piece with the lens element. The shape of thereading field may be as desired, circular or other conformation and thesegment may be in one piece or of a plurality of sections as desired. Inthe instance of the use of a. plurality of sections it is to beunderstood that the said sections may be formed of a different index ofrefraction or to different surface curvatures to produce varying focalpowers and/or size corrections throughout the various sections. Theconstruction and arrangement of this field may be in accordance with anyof the reading fields known in the art and may be placed in any desiredrelation to the distance field as well known in the art. The segmentfield may be applied .to a full sized lens or to a reduced sizedlenticular lens as well.

In Figure II the distancefield is comprised of the lens elements 5 and 6held apart by the spacer member I. This field includes in its opticalcorrections the means for changing the size of image without change offocus in combination with the required correction for sphere, cylinderand prism, as the case may be, through the relationship ofthe opticalsurfaces placed on the elements 5 and 6. This field then has power andmagnification of a required amount. In the element I is placed therecess 8 having an optical surface and on the element 5 and in line withthe recess 8, the raised optical surface 8. The surfaces 8 and 9 arerelated optically to give a required power and magnification differentfrom that of the distance field. As shown the magnification will beincreased. The optical surfaces 0 and O are of optical conformations andstructure well of a "size lens is the distance of the object from knownin the "art for producing the required optical properties. The part 9may be integral with 5 or a separatepiece cemented or fused thereon. Theresultafnt'lens will produce a distance field of I required powerand"niagnifirmtion and a reading field of required power andmagnification different from'the distance field.

cation the system is calculated just as the prior art lens. If it isalso to have size magnification for a given distance o'f object it iscalculated by the method'set' forth by th'e'said Ames patent. Bothcalculations are prior art procedures. As

shown'thefllens systemsrepresented'by 8 and 9 have both a differentfocal power and a different "size" magnification from the major field.

The procedure is to first calculate the major field to required powerand s ize magnification to the required distance of object and thento'calculate the minor field to required focal power and size"magnification to required distance of object and then produce the majorlens and impose the minor one thereon. Both fields are calculatedby'priorartmethodsas stated, and the surfaces are made and the lensconstructed by well known prior art'methods of grinding and polishing.The surfaces are of types well known in the'art.

The invention is new and novel in the calculated relationship ofthelens' elements to produce size magnifications at different distancesand in the result obtained. The process comprises a new series of stepsto so relate'the parts for the desired results. patent in theformula-therein, one of the elements the eye. The majorlensis' primarilyfor distant vision, i. e., for sight of a relatively distant ob-' ject.The minor lens is primarily for reading distance or distances nearerthan the object of the major field, hence the distances being differentfor the two fields the size magnification for the two fields may also bedifferent and their ratios different. H b

In considering the focal powers of the two fields as well as'the sizemagnifications therefor, it is .to be understood that as usual in priorart lens considerations, zero power is the transitiori point from plusto minus powers, and zero magnification is the transition point fromplus to minus magnifications. Zero, therefore, is consider'ed a power ormagnification as the case may be, to minus powers and magnifications.

The lens of Fig. IIIfwill give the same results as the lens of Fig. IIand is the same in strucv ture, except that the segment I0 is an insertof a glass" of differentindex of refraction from the glass of element. 5operating on an optical principle well understood in the art to producea different power from the element 5. The insert l0 may be cementedorfused in the recess in the element 5. The curve of the recess isoptically arranged and constructed.

In Figs..II and III the reading field may be made withthe same power as'the 'power of the field 'of required power and magnification and areading field of required power and magnification butdifferent from thatof the distance field. In this lens, the magnification will bedeereased.

As set forth in the said Ames provide the transition between plus andThis lens is just the reverse of the lens of Fig;

11. The segment 9 is placed on the element 6 and surface 8 on theelement 5. In this lens, the

reading field may be made the same power as the distance field and onlythe magnification of the reading field be changed, if desired,

'It is apparent that with the four lens'surfaces of the eleme ts 5 and 6and theadditional surfaces of the s gments 9 and I0 and the recess 8.1 agreat latitude is afforded in making optical combinations for theregular corrections for sphere,

cylinder. and prism as well as forchange in magnification without changein focus.

It is to be understood that the separate iens elements may be made ofglasses of any desired 15 indices of refraetionand that the saidelementsmay bese'cured together by uniting their entire peripheral edges or byuniting the said edges only,

at a few selected spots. This uniting depends, 20

largely upon the h pes and curvatures Qf th elements.

In Fig. VI is shown a lens comprising two elements 20 and 2|, having thesurfaces" 22,,23, 24'

and 25. The surfaces'23 and 24 are ,comating' and the two elements arefitted together on these held together. I ment 2|, is made the recess 26and in this recess .25 surfaces. being cemented, fused, or otherwise 1:

In the surface 24, of the eleis fitted the sOgmChtZ'l. This segment isof a dif- I ferent index of refraction from 2| and is cemented, fused orotherwise secured in the recess 26.- The segment is designed to give thereading I poweigand the surfaces 22 and 25'm'a'y be related to give thedesired focal power and include the correction'desired for magnificationor change of size of image.

.35" This'arrangement permits 'the,,;

making of desired surfaces on the faces 22 and;

25 independent of the segment. These surfaces may be either spherical ornon-spherical as re:- quircd to give the required power and magnification. This arrangement permits the use of toric or aspherical curves oneither of these surfaces whereby the lens may be corrected for marginalastigmatism, as well; if desired. The outer 'sur' I faces are entirelyindependent of the surfaces of Although the segment 21 has been" thesegment. described as being secured in a recess 26 formed in the element2|, it is to be understood that the said segment may be secured in'arecess in the element 20 if desired or in recesses formed in;

' of pieces of lens medium of different indices of refraction, it isquite obvious that fields of different focal powers may be obtained. Theconstruction of the lens, indices of refraction of the various parts ofthe Ienssurface curvatures, etc., of course, will depend upon thedesired characteristics of the lens. b

While the above lenses have been described as being formed of twoseparate elements 5 and'6 or 20 and 2|, as the case may be, it is to beunderstood that lenses having the same characteristics as regards powerand magnification in the different focal fields thereof may be formed,as shown in Figs. VII and VIII, from a single pieceof lens medium havingeither the reading field formed integrally therewith or by fusing ,orotherwise securing a separate segment, of glass thereto.

In substance, the said lenses will be in effect similar to the lensesshown in Figs. II to IV,

inclusive, except that the filler piece I is in effect removed and theportions 5 and 0 formed integral with each other.

As previously stated, it has been found by actual tests that an eye hasa different magnification or image when looking at a near object atsubstantially reading distance from the eye than when looking at adistant object or object twenty feet or more from the eye, so that whenforming a bifocal or multifocal lens this difference of image size forsaid distances must be considered as well as the focal power for saiddistances. It has been found that the focal power differences betweenthe said fields may be greater or less than the size image difierencesand that each factor must, therefore, be considered and dealt withseparately in the computation of the lens. So far we have spoken ofmagnification and size magnification in the general known procedures.For the actual discussion of Figures VII and VIII it is advantageous touse the concepts of my copending application Serial No. 720,594 whichhas become Patent No. 2,077,134. This concept briefiy is thatmagnification due to the focal power of the lens system can be separatedfrom the magnification due to the shape, thicknesses, etc. of the lenssystem; the first of thesewe call power magnification and the lattershape mangnification.

In Fig. VII there is shown a one piece type lens having a correction fora distant object 30 at a large distance A from the eye and an object IIat a distance indicated at B substantially equal to the readingdistance, usually four hundred millimeters from the eye. This particularlens is formed from a crown glass having an index of refraction of1.523, which is in common use in forming present day ophthalmic lenses,and has all of its surface curvatures controlling the power and/or sizecorrection in the different focal fields thereof, formed on a singlepiece of lens medium.

To illustrate how a lens of this nature may be formed with all theprescriptive requirements as to focus and magnification, let us firstassume that the finished lens is to have a power of plus 1.00 dioptersand a shape magnification of 2 per cent in the distance portion thereofand a plus 2.00 diopter addition or 3.00 diopter power and a shapemagnification of 4 per cent in the reading field.

In the following computations:

D1 is the front surface of the lens. D2 is the rear surface of the lens.

Dc is power as measured from the ocular surface, assuming parallel lightentering the system. 8 is the reduced thickness of each element. 'r isthe actual thickness of each element.

21 is the total lens thickness. 11 or a is the glass index,

M is the total magnification or size magnification of a distant object.

S is'the distance shape magnification. P' is the distance powermagnification.

U is the distance from the effective stop point to the ocular surface ofthe lens system.

d is the distance from the ocular surface to the object.

M is the total magnification of a near object.

S is the near shape magnification.

P is the near power magnification.

p1, p1, etc. are the surface powers of successive surfaces following thesign conventions of Pendlebury as given in Charles Pendlebury, M. A.FRAS published Cambridge, England, 1884."

t1, 152, etc. are the successive thicknesses or separations divided bythe index of refraction of the medium, either glass or air, and taken inthe negative senses as Pendlebury uses these values.

The terms A, B, C and D are the Gauss equations as given by Pendlebury,designating certain function of surf es, indices, thickness and specialrelations in a lens system. Specifically A is the reciprocal of theequivalent focal length of the system, B is the partial derivative of Awith respect to the first surface power, C is the partial derivative ofA with respect to the last surface power and D is the second partialderivative with respect to the first .and last surface power.

l is the distance for the entrance window or reference point of the eyeto the object and is equal to U+d'.

The formulae which will be used in computing the reduced thickness 3from the fictitious ocular surface (D2) and a given value of S'-1 or thepercentage of S is given implicitly in my copending application but forclarity the derivation of this form is here given. Using the notation ofthe copending application, for a single thick lens Then if we make p2such a value that De is zero we have Pi i *m-Pzfi-( Therefore, S'1=zt1=s(Dz) where (D2) is the fictitious ocular surface.

The derivation of the equations in the above paragraph are given inapplicant's copending application Serial No. 720,594, filed April 14,1934, which has become Patent No. 2,077,134.

The method of computing the lens is as follows:

I first assume a fictitious zero power lens ocular surface 32 of sayminus 6 diopters which surface will be generally known in the formula asthe fictitious rear surface (D2) of the lens. The minus 6 diopter curveis chosen because, for a weak distance correction, it enables theproduction of a good form of lens. With the known required values forthe distance portion of the lens, the surface curvatures, thickness,etc. of the lens are applied as follows:

thickness of lens=s index=0.00508 m.=-=5.08mm.

The front surface 33 to give zero power will therefore be plus 5.88diopters. To obtain the desired power of plus 1.00 diopter in thedistance portion, I add this value to the fictitious minus 6 surface andobtain an actual minus 5 diopter surface. The distance lens form willnow be; on the front surface 33 a curvature of about plus 5.88 diopters,a thickness 34 of about 5.08 millimeters,'and an ocular surfacecurvature 35 of about minus 5.00 diopters. Note: Actulens, would givezero power.

ally this-lens'would be a little 'better for oblique aberrations if thecomputation were gone throughagain and about/a minus-6'.50 diopters orminus 6.75 diopters fictitious surface were assumed.

Now let us assume a plus 2 diopter addition for segment field-with a 4per cent S magnification, that is, S'-1:0.04. This 2 diopter additiongives, for the total reading power, plus 3 diopters. The segmentdiameter is to be 20 millimeters.

The exact glass thickness 43 depends upon the position and diameter ofthe segmentand for each specific case must be "determined by 'the usualmethods of determining lens'thicknesses. However, for fairly closeapproximation, to start with, we can assume certain obvious things.

First" 'of all, the segment has two diopters more power than thedistance so' it will be thicker than the distance portion where thesegment is placed. This increase in thickness is 0.2 millimete'r but thesegment is below. the center of the distance portion wherethe lens isthinner; now .a fair position is for the upper edge to be 5 millimetersbelow the optical center of the distance portiomor the optical center ofthe distance portion to be 15 millimeters from the center of the-segmentcausing the center of the segment to be 0.2 millimeter thinner thanthecenter of the distance portion so'our new thickness 43 at center ofthe segment will be I 5.1 mm.+0.2 mm.0.2 m .=5.1 mm =-.0.0051 m.

The next thing to obtain is a fictitious curvature 36 by" whichthe powerand magnification of the reading field can be worked out. We requirethat;(S'-1):4'7 :0.04:-s (D2) and 0.04 4- (D2)-' m- 12 diopters for thefictitious ocular surface 36. "Now we desire a total power of plus 3diopters, therefore "the actual ocular surface 37 of the segment is-12+3:-9 diopters andthe front curve 38 of the segment is then put on,which if we actually had the fictitious surface -l2 diopters on the beabout plus 11.5.4 diopters. .These figures for the correction; of powerdue to thickness and the depth of curve or thickness change fordifferent powers have been taken from practical lens a grinders chartsand are simply approximate and are given herein only by way ofillustration.

Let us take the same example as the preceding and assume no shapemagnificationSf for the segment. This'is obtained by makingv the front38 fiat where the segment isand putting all "the power on the ocularsurface 31, thatis, for

the segment a .plus 3.00 diopter-curve instead of the :minus 5.00diopter curve of the distance power. v

It. will be seenthat any values within practical limits as regards powerandshape magnification may be obtained by the method shown.

Now if it is desired to make a fused bifocal, as

shown in Fig. VIII, the computation is longer because there are threesurfaces involved, that is, the-front-surface 39, the interface 40 ofthe fused combination and the ocular surface 41, 39 and (H correspondingto 33 and:=35-respectively of Fig. VII. Y c: I:

We willtake the same distance correction and actually the same-distancelenses that 'set forth This surface will 1 above in Fig. VII but becauseof the aberrations introduced by a too steep interface 40 we will take amore common value of the'magnification, that is 3%, instead of 4% forthe reading.

While the equations for this may be written out and-solved directly, anindirect approach is bet ter. i

The total glass thickness 44' .will be for a first approximation0.2'millimeter less'than the thickness 34 of the distance portion,thatis 4.0 millimeters, because the distance lens is thinner where thebutton is. Then, if this were a one piece lens we would have afictitious ocular curve (D2):0.03 (i. e. 3%) divided by the reducedthickness which is (D2) :9.31 diopters I Theone piece front curve 33would then be plus 9.02 diopters instead of plus 5.88'or we will'have tomake a fused button 42 of about 3.25 diopters addition to correspondwith the portion '38 in Fig. VII.

Now. assume high index flint usually used for additions of over 2diopters, that is, 1.700 flint ocular surface 45 (-.-9.31 +3.00) or--6.31

diopters. I

From one of my copending applications Serial 2,077,134, we have SI D Dapproximately where D1 or front surface 39 is 5.88 X W 188D I D2 orinterface is 3.72 X 'm -+4.98D

Dsor countersink surface is 3.72 D s1 or reduced thickness of segment:

1'2 or space between Dzand D3=0.0000 sa or thickness between D3 and D4:

\ .004 m --.0O26?r -s1D1: 0.0046 --.93 (D1+D2+D3) :0.0240 sum: -0.0286

therefore or S':1.03 which is 3% shape magnification. We have shownexamplesof the computation which has become Patent No.

by the method which follows:

The shape magnification for a distant system S can be expressed in termsof the near shape magnification S and the known lens constants by asimple formula. In my copending application Serial No. 720,504, whichhas become Patent No. 2,077,134, I have shown that the totalmagnification of a system is U+ Z'r-I-d l uaa-manca- U(BAd)+Cd-D Then,if we consider the power of the system as in one lens surface at theposition of the ocular surface and call this value D=A/C', all the othersurfaces vanishing, then A= 1; C=1; B=l; D=0, and d, the distance fromthe front of the system becoming d which is the new d; and since theobject is at the same position in space d=d+2-r At the same time, callthis specialized value of M as P since it is the magnification due topower alone measured from the ocular surface. Substituting these valuesin the above equation we have for the power magnification for a nearobject I I Then for any shape of system, define 8 so that SXP=M, thetotal magnification therefore I S= M( D.)

. 5 and I- UD,

which of course, is easily derived from the near value of P since d d Td'+U and if d becomes very great the ratio of d! T is unity.

The ratio of the distance to the near shape magnification s c.M( whichafter reduction becomes (zr+ DS")(1 UD.)

u The first term involves all known quantities except S' -I and for thisthe value of 54-4 may be used as a first and generally finalapproximation. The distance from the effective stop point to the ocularsurface is U. The distance from the ocular surface to the object is dand I=U+d' and of course De is the power as measured from the ocularsurface assuming parallel light entering the system.

An approximate computation of the values of this term follows: The valueof U may be about millimeters or 0.020 meter. Value (1' is for ordinaryreading distance, 400 millimeters=0.400 meter. This corresponds to theusual 2.50 diopter addition for reading.

We will take a total of plus 3 diopters for Do.

Now, let us assume a value of S or S as 1+3% or 1.03. Then, thefirstterm is:

M20 1.o31 =l(o;og a o 3 0-420 10.020X(%)X3 21 0.94 Therefore. this termadds 03% to S to obttain s'. The next term involves the thickness of the-o.oosa. I u Therefore, we have, for the second term.

=-0.004 approximately, or about 0.4%, as the second term in thereduction of or, in per cent we have S differs from S by 0.1% for thisexample. Therefore, for this example. which is an average one, noreduction is needed for S to S' but when needed the values can be easilycomputed or tabulated.

In the designing of the lens an average thickness value whichcorresponds to a reading addition 'of about two diopters is selected sothat if the required addition is from 1.50 to 2.50 diopters, the errorintroduced due to variation in thickness brought about by the use ofthese additions will be relatively small and negligible. Thisarrangement permits the provision of lens blanks by which a certainshape magnification for the major field and another shape magnificationfor the segment may be obtained for a plurality of lenses with varyingpower corrections in both the distances and reading portions of thelens, which power corrections will have no eifect upon the shapemagnification factor of said fields.

The latter matter of computing lenses relating to my copendingapplication Serial No. 720,594, which has become Patent No. 2,077,134,provides a blank having its ocular side designed to receive a compoundsurface, which surface over the distance portion of the lens is such asto produce the required power through said distance portion and whichsurface over the reading area of. the lens is such as to produce thedesired power through the reading portion so that both the near anddistance prescriptive powers may be obtained independent of themagnification factors of the fields. This is due to the fact that inboth the distance and reading fields the shape magnification values areindependent of the power magnification values of the respective focalfields of the lens.

It is to be understood that although I; have shown and described, inFigs. VII and VIII, a one piece and fused type bifocal of the characterdescribed, the distance or major portion of the lens maybe formed in amanner similar to a single vision lens element having continuoussurfaces, thereon producing the desired magnification factor and focalpower factor for the distance field of the finished lens and that .I maythereafter cement or otherwise secure a segment to either the front orrear surface of said element or may secure segments to both the frontand rear surfaces of said element to produce areading or near visionfield having the required magnification factor and focal power factorfor the near object distance produced by forming the required opticalsurfaces on said segment or segments. In this instance, the surfacecurvatures and thickness of the different focal fields will be obtainedin a manner similar to the lenses of Figs. VII and VIII.

From the foregoing description it will be seen that I have providedsimple, efficient and economicai means whereby a lens comprising two ormore focal fields may be provided with any desired focal power andmagnification correction whereby said focal power and magnificationfactors may be considered and controlled more or less separately andindependently of each other.

Having described my invention, I claim:

1. A lens having surfaces of different curvatures providing differentfields for equalizing the size difference of images of the two eyes fordifferent given object distances, each having prescriptive shapemagnification and prescriptive focal power for said given objectdistances and a given position before the eye, comprising lens medium ofgiven index of refraction forming the major field of the lens, and aminor piece of lens medium of a different index of refraction secured tosaid major field and contributing to the formation of the minor field ofthe lens, said major field having a front optical surface and athickness which combined together for a lens medium of said index ofrefraction and given position before the eye will produce theprescriptive shape magnification desired of said field, said minor fieldcomprising pieces of lens medium of different indices of refractionsecured together along contiguous optical surfaces, the said minor fieldhaving a front optical surface thereon which when combined with thecurvatures of said contiguous surfaces, thickness of said field, and therespective indices of refraction of said mediums will produce the shapemagnification desired of said minor field and each of said fields havinga rear or ocular surface of such power that when combined with the pthersurfaces of said fields, thicknesses of lens mediums and indices ofrefraction of said lens mediums will produce the prescriptive focalpowers desired of said fields with substantially no change of the shapemagnification factors of said fields.

-2. A blank for a lens having surfaces of different curvatures providingdifferent fields for equalizing size difference of images of the twoeyes for different given object distances, each having prescriptiveshape magnification and prescriptive focal powers for said given objectdistances and agiven position before the eye, comprising lens medium ofgiven index of refraction forming the major field of the lens and aminor piece of lens medium of a different index of refraction secured tosaid major field and contributing to the formation of the minor field ofthe lens, said major field having a frontoptical surface and a thicknesswhich combined together for a lens medium of said index of refractionand given position before the eye will produce the prescriptive shapemagnification desired of said field, said minor field comprising piecesof lens medium of different indices of refraction secured together alongcontiguous optical surfaces, the said minor field having a front opticalsurface thereon which when combined with said curvatures of thecontiguous surfaces, thicknesses of said field, and the respectiveindices of refraction of said mediums will produce the shapemagnification desired of said minor field and an excess of material inthe direction of the thickness on the ocular side to provide for theplacing on said ocular side of optical surfaces over each of said fieldsof such powers that when combined with the other surfaces of said.fields, thicknesses of lens mediums and indices of refraction of saidlens mediums will produce the prescriptive focal powers desired of saidfields with substantially no change of the shape magnification factorsof said fields.

,3. A spectacle lens system for use in combination with anotherspectaclesystemfor the other eye, for equalizing the size difierence ofimages of the two eyes for different given object distances, havingsurfaces of different curvatures providing two fields, one adjacent theother, each field having prescriptive shape magnification and aprescriptive focal power for each of the said given object distances anda given position before the eye, comprising lens medium of given indexof refraction forming a major field of the lens and a minor piece oflens medium of a different index of refraction secured to'said majorpiece and contributing to the formation of the minor field of the lens,said major field having a front optical surface and a thickness whichcombined together for a lens medium of said index of refraction andgiven position before the eye will produce the prescriptive shapemagnification desired of said field, said minor field comprising piecesof lens medium of different indices of refraction secured together alongthe contiguous optical surfaces, the said minor field having a frontoptical surface thereon which when combined with the curvatures of saidcontiguous surfaces, thickness of said field and the respective indicesof refraction of said mediums will produce the shape magnificationdesired of said minor field and each of said fields having a rear orocular surface of such power that when combined with the other surfacesof said fields, thicknesses of lens mediums and indices of refraction ofsaid mediums will produce the prescriptive focal powers desired of saidfields with substantially no change of the shape magnification factorsof said fields.

EDGAR D. TILL-YER.

